1. Field of the Invention
The present invention relates to a method for fabricating a cantilevered type film bulk acoustic resonator and a film bulk acoustic resonator fabricated by the method.
2. Description of the Related Art
As mobile communication devices, such as portable telephones, become increasingly prevalent, demand for compact, lightweight filters used in such devices is also rising. Film bulk acoustic resonators (FBARs) can be used as such compact, lightweight filters. FBARs can be inexpensively mass-produced and can be implemented in a micro-structure. A FBAR has a multi-layer resonance section in which a bottom electrode, a piezoelectric layer, and a top electrode are sequentially provided in this order. The FBAR uses a piezoelectric phenomenon, by which when electric energy is applied to the top and bottom electrodes, piezoelectric effect is produced and resonance results. In such a FBAR, separation between the substrate and the multi-layer resonance section is needed, so that acoustic waves generated from the piezoelectric layer are not affected by the substrate. In a Bragg reflector type FBAR, the separation is achieved using a reflection layer. In an air gap type FBAR, the separation is achieved using an air gap.
The Bragg reflector type FBAR shown in FIG. 1A is a FBAR adapted to generate resonance, in which the FBAR includes a reflection structure 11 deposited on a substrate 10 and a resonator on the reflection structure. The resonator includes a first electrode 12, a piezoelectric material 13 and a second electrode 14. The reflection structure 11 typically includes a plurality of dielectric layers alternating between low impedance and high impedance materials to insure efficient confinement of the acoustic energy in the resonator. Thus, such a Bragg reflector-based FBAR includes a reflection structure 11 having large acoustic impedance difference therebetween disposed in multiple layers on the substrate 10, which induce Bragg reflection to resonate due to acoustic waves between first and second electrodes 12 and 14. However, the precise thickness control of the layers of the reflection structure 11 for the total reflection in such a Bragg reflector type FBAR is not easy and increases the manufacturing cost.
An example of a bulk micro-machined air gap FBAR is shown in FIG. 1B. This membrane-based FBAR includes a silicon oxide layer (SiO2) deposited on a substrate 15 forming a membrane layer 16 on the reverse side of the substrate 15 through a cavity 15′ formed by isotropically etching the substrate 15. A resonator includes a first electrode 17 formed on the membrane layer 12, a piezoelectric layer 18 on the lower electrode 17, and an upper electrode layer 19 on the piezoelectric layer 18. However, the fabrication of the membrane is difficult and the devices are frequently damaged during the etching process of substrates thereof.
An example of a surface micro-machined air gap FBAR is shown in FIG. 1C. This surface micro-machined FBAR includes an air gap 25 formed through a sacrificial layer on a substrate 20 using micromachining technologies and has a membrane layer 21. A resonator is provided on top of the membrane layer 21. The resonator includes a first electrode 22, a piezoelectric substance 23, and an second electrode 24. The resonator is formed by providing a sacrificial layer on the substrate 20, fabricating a resonating multi-layer resonance section on the sacrificial layer, and then removing the sacrificial layer through a via hole. However, this fabrication process is complicated, and collapse and peeling off of the structure may result during the process.
An example of a cantilevered type FBAR is shown in FIG. 2. The cantilevered type FBAR includes an abutment 31 on a substrate 30 on which the resonator is formed. The resonator includes a first electrode 32, a piezoelectric material 33 and a second electrode 34. The cantilevered type FBAR may be fabricated by depositing a sacrificial layer, depositing a SiO2 layer to form the abutment 31, sequentially laminating the first electrode 32, the piezoelectric material 33 and the second electrode 34 on the abutment 31, and then etching the sacrificial layer. However, this fabrication process is complicated because the process includes laminating and patterning the SiO2 layer. Further, since the SiO2 layer is positioned under the first electrode 32 as the abutment 31, some acoustic wave energy generated in the piezoelectric layer 33 is absorbed by the SiO2, thereby lowering the quality factor Q (Q-value) of the FBAR.
Further, wire bonding must be used to connect the first and second electrodes 32, 34 in FIG. 2 with external terminals. It is difficult to precisely control the resistance of these connections. Also, the separate connection required for each FBAR do not facilitate using a plurality of FBARs to form a more complex structure, such as a filter.